The present invention relates to a method of producing pure silica from a crude silica feed source, and more particularly, to a method in which the silica feed source is reacted with fluosilicic acid to produce an ultra-pure silicon tetra-fluoride by-product.
Amorphous silica is an important industrial product, for which several industrial processes have been developed. The chemical handbook “Gmelins Handbuch der anorganischen Chemie”, Vol. 21 (1928), p. 861, discloses a process of Liebig (1857) in which silica is autoclaved in the presense of an alkaline solution. Notable among the numerous publications and patents relating to silica production is U.S. Pat. No. 4,336,235 to Deabriges, which teaches a method for the production of sodium silicate involving the calcination of sand with sodium carbonate at 1,500° C., followed by the dissolution of the solid mass in an autoclave, to obtain a sodium silicate solution. By treating this solution with hydrochloric acid, an amorphous silica product with a high surface area (200 m2/g) is obtained. The product is suitable as a filler for rubber and plastics. In another silica production process, fluosilicic acid (or a salt thereof) is hydrolyzed with alkali, whereby amorphous silica precipitates from fluoride solution. In the above-described processes, the amorphous silica product is contaminated by impurities present in the mother liquor.
U.S. Pat. No. 6,312,656 to Blackwell et al., describes other patents concerning the production of silica, and particularly pure amorphous silica, from siloxanes. It is emphasized by Blackwell et al., that some processes are costly, because of the use of siloxanes as raw material, but yield products that are suitable for the production of high quality glass, especially optical glasses. The raw material in this process is purified polyalkyloxane, from which the silica is obtained by thermal decomposition. Another process in which silicon tetrachloride by hydrolysis is used, produces very pure silica, but this process is also very costly: For the production of one ton of silica, approximately three tons of expensive silicon tetrachloride are needed.
U.S. Pat. No. 5,853,685 to Erickson discloses a process for producing high purity silica from waste by-product silica and hydrogen fluoride. The high purity silica is obtained by the reaction of impure by-product waste silica with hydrogen fluoride, preferably in the presence of water or sulfuric acid, producing silicon tetrafluoride gas and a mother liquor. The silicon tetrafluoride is separated from the mother liquor, which retains the impurities originally contained within the impure silica. The silicon tetrafluoride gas is contacted with high-purity water, in a clean environment, to form a slurry of high purity silica and high-purity hydrofluosilicic acid. A portion of the silica is filtered from the slurry and washed, producing a high purity silica product. The rest of the silica-hydrofluosilicic acid slurry is preferably reacted with ammonia to form a slurry of ammonium fluoride and silica. The silica is separated from the ammonium fluoride and preferably washed and calcined to remove any remaining ammonium fluoride, leaving additional high purity silica product. The separated ammonium fluoride may be reacted with lime to produce additional products for recycling back into the process.
There are several deficiencies with the art taught by U.S. Pat. No. 5,853,685. The reaction with ammonia, separation of ammonium fluoride, and ammonia recovery stages are capital-intensive and energy-intensive, and introduce various safety concerns into the industrial process.
Moreover, as taught, the process requires the costly addition of both hydrogen fluoride and lime, as well as an ammonia make-up stream. The generally infeasible process economics are acknowledged by U.S. Pat. No. 5,853,685 to Erickson:                Under current economic conditions, the price of hydrogen fluoride, at approximately $550.00 per ton, may make it economically impractical to purify silica in the above-described manner in most situations. The cost for the raw materials alone would be approximately $1.10 per pound. However, where low cost hydrogen fluoride is available and fluosilicic acid can be reused, the process of the present invention is economically feasible, as the cost of the pure silica is approximately $0.11/lb.        
There is therefore a recognized need for, and it would be highly advantageous to have, a process for producing high-purity silica that that is more simple and economical than the processes known in the art, and overcomes the manifest deficiencies thereof. It would be of further advantage to have a process that can utilize various low-grade sources of silica to produce the high-purity silica.